The Mechanisms of Sulfur-Containing Metal Complexes as Uv-Stabilisers

Abstract

The antioxidant activities of some sulfur-containing compounds have been studied in polyolefins (polyethylene and polypropylene) and in model systems initiated by alkoxyl radical generators (hydroperoxides). The chemical reactions involved in the antioxidant behaviour of nickel complexes of dithiophosphoric and xanthic acids were investigated both by kinetic methods and product analysis. Two mechanisms were identified: the First is a radical trapping process involving the metal complex itself and the second is a Lewis acid catalysed destruction of hydroperoxides by a transformation product(s) of the metal complex and not by the complex itself. The extent of the participation of each of these processes was found to be a Function of the hydroperoxide/antioxidant ratio: et low molar ratios, the First process predominates. Disulfides derived from the above complexes were found to be the initial oxidative products of the metal dithiolates and these were also investigated. The main mechanism of their antioxidant action involves the decomposition of hydroperoxides through their oxidation products, by a catalytic ionic process. Their efficiency depends very much on the availability of oxygen. The catalytic Function is enhanced under conditions of severe (excess oxygen) processing of disulfides in the polymer. Extensive photo- and thermo-oxidative stabilisation of polyethylene and polypropylene was achieved when the sulfur-containing metal complexes, and their corresponding disulfides, were incorporated in the polymer. Furthermore, synergistic combinations with the commercial UV-stabiliser, 2-hydroxy-4-octyloxybenzophenone, has led to a superior performance when compared to commercially available UV-stabilisers.